From the Guidelines
The workup of Cushing syndrome should begin with establishing the diagnosis through screening tests, including 24-hour urinary free cortisol measurement, late-night salivary cortisol testing, and the overnight 1-mg dexamethasone suppression test, as recommended by the most recent consensus guideline update 1. The initial step in the workup of Cushing syndrome is to confirm the diagnosis through screening tests.
- The screening tests should include 24-hour urinary free cortisol measurement, late-night salivary cortisol testing, and the overnight 1-mg dexamethasone suppression test.
- At least two abnormal results from different tests are needed to confirm the diagnosis, as stated in the guideline update 1. Once Cushing syndrome is confirmed, the next step is to measure ACTH levels to differentiate ACTH-dependent from ACTH-independent causes.
- For ACTH-dependent disease, an 8-mg dexamethasone suppression test and CRH stimulation test help distinguish between pituitary (Cushing disease) and ectopic ACTH sources.
- Imaging studies are then used based on biochemical results: MRI of the pituitary for Cushing disease, and CT/MRI of the chest, abdomen, and pelvis for ectopic ACTH syndrome. For ACTH-independent Cushing syndrome, adrenal CT or MRI is indicated to identify adrenal tumors or hyperplasia.
- Bilateral inferior petrosal sinus sampling may be necessary when biochemical tests and imaging are discordant, particularly to confirm a pituitary source. Throughout the workup, it's essential to exclude exogenous glucocorticoid use and consider cyclical Cushing syndrome, which may require repeated testing during symptomatic periods, as noted in the guideline update 1. This systematic approach helps identify the appropriate treatment strategy, whether surgical, medical, or radiation-based, and is supported by the most recent consensus guideline update 1.
From the FDA Drug Label
The FDA drug label does not answer the question.
From the Research
Diagnostic Approach to Cushing Syndrome
The diagnostic workup of Cushing syndrome involves a stepwise process that includes screening and confirming the diagnosis, as well as establishing the etiology of the syndrome 2, 3. The following steps are involved in the diagnostic approach:
- Screening tests:
- Urinary free cortisol measurements
- The dexamethasone suppression test
- Late-night salivary cortisol or midnight serum cortisol measurements
- Confirming the diagnosis:
- Measurement of ACTH to discriminate between ACTH-dependent and non-ACTH dependent causes of Cushing syndrome
- Establishing the etiology:
- Adrenal imaging for non-ACTH dependent forms
- Pituitary MRI imaging, the high dose dexamethasone suppression test, the CRH test, bilateral inferior petrosal sinus sampling, and imaging modalities to detect ectopic ACTH secreting lesions for ACTH-dependent forms
Screening Tests
The screening tests for Cushing syndrome include the 1-mg dexamethasone suppression test, 24-hour urinary free cortisol excretion, and late-night salivary cortisol measurement 3, 4, 5, 6. These tests have their own limitations and pitfalls, and require careful interpretation.
- The 1-mg dexamethasone suppression test and 24-hour urinary free cortisol excretion demonstrate high sensitivity and specificity for the diagnosis of Cushing syndrome, but each test has its own distinctive features, making it preferable in specific clinical conditions 6.
- Late-night salivary cortisol measurement is an accurate and reproducible test with a high sensitivity for Cushing syndrome, but its performance in mild Cushing syndrome has not been reported 4.
Establishing the Etiology
Once the diagnosis of Cushing syndrome is established, the next step is to establish the etiology of the syndrome.
- Measurement of plasma ACTH concentrations differentiates between ACTH-dependent (80%-85%) or ACTH-independent (15%-20%) causes 3.
- Dynamic tests, such as the CRH test and high-dose glucocorticoid feedback, and imaging procedures, such as pituitary MRI and adrenal imaging, are useful for the differential diagnosis 2, 3, 5.
- Bilateral inferior petrosal sinus sampling is considered the gold standard for establishing the origin of ACTH secretion, and is recommended in patients with ACTH-dependent Cushing syndrome whose clinical, biochemical, or radiological studies are discordant or equivocal 5.